As is well known, porcelain fused to metal (PFM) dental casting alloys should provide a high degree of biocompatibility or inertness to the conditions in the mouth and good physical properties so that they will provide long lived usage. In addition, they must provide good bonding characteristics to the porcelain coatings and other characteristics which are compatible with the porcelain coatings, such as a similar coefficient of expansion and good bond strength. Lastly, the alloy should process well during casting and be usable with commercially available porcelains.
As set forth in ADA Specification No. 38 approved in May 1991 and effective 1992, PFM alloys should exhibit a desired balance of physical and mechanical properties. To withstand the stress transmitted through the restoration, the alloy must have an offset yield strength at 0.2 percent of over 250 mPa. Tensile elongation of at least 2 percent is required for proper dental manipulations.
The hardness must exceed 150 Vickers to withstand the abrasion of opposing teeth. Moreover, a dental casting alloy must be able to be soldered before the porcelain firing cycle. Since porcelain is fired at approximately 1000.degree. C., the alloy must exhibit a solidus temperature of at least 1100.degree. C. to allow the solder to flow without starting to melt the casting. However, in order to allow the alloy to be cast with standard equipment found in dental laboratories, the liquidus temperature must not be greater than 1400.degree. C.
For many years, gold/platinum and gold/palladium based alloys had been preferred for dental castings because they had a highly desirable balance of the above properties. Commercially available dental porcelains were formulated so as to be compatible with their thermal expansion coefficients.
After the cost of gold and platinum escalated, there were extensive efforts to find alternate noble metal alloy compositions which would afford acceptable properties at considerably lower cost. Base metal alloys were found to suffer from one or more limitations such as lack of sufficient biocompatibility, undesirable aesthetics, etc.
As an alternative, gold alloys containing large amounts of palladium and other metals and palladium/silver alloys were developed. The latter simulated the appearance of platinum and "white" gold alloys as a substrate for porcelain and provide a high degree of biocompatibility while still exhibiting useful casting and physical properties. Generally, the silver had a tendency to migrate at the porcelain firing temperatures and to discolor the porcelains being fired thereon. Illustrative of these alternative alloy approaches are German U.S. Pat. No. 4,205,982 granted Jun. 3, 1980 and Schaffer U.S. Pat. No. 4,350,526 granted Sep. 21, 1982.
In Schaffer U.S. Pat. No. 4,387,072, there is disclosed a novel palladium based PFM alloy which substantially eliminated the problem of discoloration in restorations. This and similar alloys have enjoyed substantial commercial success as PFM alloys.
However, the cost of the high palladium content in such alloys is relatively high as compared with the earlier palladium/silver alloys. Thus, there has remained a need for a lower cost PFM alloy having the desired nobility and producing desirable dental restorations.
It is an object of the present invention to provide a novel palladium/silver based dental alloy which exhibits a highly desirable balance of properties, including biocompatibility, and good bonding of the porcelain coatings fired thereon.
It is also an object to provide such an alloy which is less costly than a high palladium content alloy.
Another object is to provide dental restorations comprising castings of such alloys with porcelain coatings fired thereon, wherein the porcelain coatings are essentially free from discoloration and exhibit a high degree of bonding to the casting.